20794198 tn?1534529493

QT/QTc INTERVAL over and under calculation

Is anybody able to understand this. I had an ecg done with a HR of 104, and a QTc of 449ms. I didnt unfortunately get to see the QT mm, just the QTc calculation. 30 minutes later, it was repeated at 85bpm, and the QTc was down to 420 ish ms. I am baffled as i thought that with the higher heart rates, the QT interval should shorten and vice versa with lower heart rates. Yet mine did the opposite? Is this a case of overcalculation on the ecg's part as my heart rate was over 100?
6 Responses
20748650 tn?1521032211
Nope, qtc does not shorten or lengthen with heart rate.

Qt interval changes.. However 'corrected' qt or 'qtc' is your qt interval after a formula has been applied to it to negate the heart rate.

QTc is not problematic unless over 500ms.

Long Qtc is caused by genetics or severe electrolyte abnormalities. It would appear neither is the case for you. That change is very much within normal limits and healthy.
Btw i almost forgot a key point.

Machine measured qtc has a significant margin of error, especially if youre in an arrhythmia like a fib or flutter.. So its very possible that your qtc changes were due to machine error.. Technology is getting better, and the machine gives a ballpark.. However the only accurate qt interval is one taken by hand.

Measuring intervals, especiallu something like a qt interval is bacically an art.. You need alot of practice and supervision before you can accurately get one.

Anyone can take an ekg class and say "yeah it looks like the p wave starts here" same applies to a q wave or a t wave.. However getting it within the 10 ms truly requires a trained eye (cardiologist of any subspecialty/ct surgeon or an rces/ceps qualified tech).. The machines estimate is just no substitute for that level of precision.
20794198 tn?1534529493
Thanks for your advice mate. Im pretty sure that the interval shortens when the rate gets higher? As QRS narrows? On some of my old ecgs, I did see one of them at 66bpm and it spat out a QT of 368ms and a QTc of 385ms. Following that was one with a rate of 85bpm and it spat out a QT of 342ms and a QTc of 406ms ?? I must be reading them wrong perhaps? I did already have a stress test in 2015 on a treadmill @ 201bpm and the e.p said everything was fine.
Again nope, not reading them wrong.. Just margin of error for machine.

You are correct that qt interval shortens with heart rate.

As for the qtc its supposed to be what the qt interval would be at 60 bpm. Its calculated as such:

Qtc= qt/ square root of (60/hr)

So at 85 qtc is calculated as such:

Qtc= 342/ square root of (60/85)
Qtc= 342/sqrt(.705)
Qtc= 342/.839
Qtc= 407

ahh im kind of with you now. so in theory, if your rate is between 60-100, the ballpark figure would be close to right on an ecg machine. but things get more complicated post 100? over correction? would you mind, if i gave you this here, seeing if the QTc was even close on one of my prior ones. HR-70BPM.... PR INTER-162MS.... QRS-86MS.... QT-352MS.... QTc-380ms
and with the mishaps on the machines, are we talking a big mis calculation? say 3 or 4ms or 15 20ms difference?
20794198 tn?1534529493
I've tried to learn the manual by hand way of it but evrytime I try I can't get the hang of it. Are you any good at it? If I was to supply you with the information on one of my ecgs?
20748650 tn?1521032211
So the margin of error is usually 40 ms or so in either direction.

For example i measured 2 qt intervals today, one interval was 40 higher then the machine one was 40 lower.

Remember each small red box on the ecg paper = 40ms... That box is 1mm wide..

So a 1 mm error in where the machine "thinks" the t wave ends can cause a huge (40 ms) shift in qt.

And yes you are absolutely correct, the higher the heart rate the more difficult it is for the machine to get a qti/qtc. This is because the p wave begins to distort the terminal aspect of the t wave as they move closer together.

As for qtc.. Im sure the machine got the formula right to convert qti to qtc, however its the qt interval input that gets messed up.

Garbage in garbage out as they say..

In terms of measuring, honestly a magnifying glass (or extremely,good eyesight), a pen, calipers and a straight ruler are key here.
ahh i see. so quite a vast amount then lol. what method would an ecg use? would it be bazetts formula?
I think its bazzets for machines.. Whatever technique uses the square root if r-r.

Any of the formulas really come up with similar results for correction though.. They only make a significant difference at really low or high heart rates. Bazzets is just the standard because its easy to remember.
All that aside though, 500 is the magic number, for both men and women, despite the variation of normal.

Qtc over 500 *OR* prlonged qtc with symptoms such as fainting warrent treatment and investigation into underlying cause (genetics or electrolytes)

If neither of these conditions are present for you, any changes you saw honestly hold no clinical value.
20748650 tn?1521032211
As for being good at it, im decent.. But unfortunately i cant just use the numbers, id need to actually see an image sorry.
P.s, I've added a photo on my page now aswell, of one of my most recent ecgs this year. Im not sure if you can view it?
I can see it.. As a thumbnail lol.. But based off just the morphology of the t wave and some squinting i can see..

1. Some slight repolarization abnormalities resulting in upsloping elevation in v2-v4. This is a normal variation, particuarly in v2 and v3, a little less commonly so in v4 but can be attributed to lead placement (slightly medial misplacement of v4 electrode), or a small and normal variation of cardiac position.

2. I cant see what lead the continual strip is (the one on the bottom). It appears to be v2 or v3. Lead II typically prints as a reference as well, alongside the abnormal lead. If v2 or v3 is not present in reference and the bottom strip is in fact lead II alone, then the possibility exists for a congealed lqt1 based off morphology. An epinephrine stress would clarify diagnosis but without symptoms or a qtc over 500ms such a test would be sort of pointless (thered be no need to actually do anything).

3. Everything else looks normal.
slight repolarizations you are correct, they are slightly apparent on all of my ecgs, maybe because ive almost always exercised since age 10?.the bottom one is lead II. the only thing i personally found odd is that, with lead II i noticed ever so slight st elevation, maybe half of one small, small box ... never noticed it before but looks consistent on my previous ones when i looked. in terms of reference for v2 or v3, im not sure what you mean lol. right hand side of the sheet it goes ... v4 ... v5 .... v6 .... then just lead II along the bottom
Yeah the slight elevation is the be expected with beningn repolarization abnormalities.. Given that what im looking at is lead 2, if that lead meets lqt criteria with that morphology then yes, long qt syndrome is a possibility... Yet as weve reiterated, under 500 and asymptomatic.. Nothing to be done. You can try an epinephrine stress test but its unlikely to change the outcome, all it will give you is the knowledge that you have it... Aside from that nothing would be done to manage it, theyd just csll it beningn and put a diagnosis code on it for administrative purposes..

The skill thats beneficial here is being able to delineate beningn from pathological morphologies. Nowhere is this true more then when assessing st changes. St changes associated with ischemia have distinctive humped or horizontal patterns. Beningn or non specific st changes tend to precede a nice smooth upslope, giving the ekg a "neat" appearence.

Ischemic st changes are concerning over 2 mm (2 small boxes), in any circumstance.

Similar logic applies to assessing long qt syndrome insomuch as im analyzing the appearence of that qt segment that occurs between the qrs complex and the t wave to help delineate what im looking at.

Childhood history has nothing to do with early repolarization in v2-v4

I can recall only a select few ecgs ive seen where absolutely no evidence of early repole was present in these leads. Its just a normal thing to see.
I see. Well last night I actually managed to successfully learn bazzetts formula myself. And I applied to 3 prior ecgs I have here at home. Two of them were the same as the heart rate was the same, aswell as the estimation of the 0.4mm boxes from q - t wave. On a third one I have of 85bpm, the ECG reports 406ms, however I measured 9 boxes on that, which equates to 429ms when applied. I seem to have what looks like a generic 9box gap between q and t through all of my ecgs, up to 90+bpm and it starts shortening.
Dont forget to figure the exact slope at baseline.. Notice how in the second example,by drawing the straight line he actually cuts off a box that appeared to be a part of the t wave due to distorition of the terminal isoelectric line.

20794198 tn?1534529493
Ok I understand, I just was baffled to see one come out at 449 and 30 minutes later when I requested them to repeat it, it had dropped to around 420, that quickly! But that probably backs your theory of saying the machine was wrong in the correcting department. I probably already know the answer to this lol, but I suppose if a reputable E.P has went through my treadmill test and holter monitor overnight, it pretty much makes it a almost impossibility to have it.
Have an Answer?

You are reading content posted in the Heart Rhythm Community

Top Arrhythmias Answerers
1807132 tn?1318743597
Chicago, IL
1423357 tn?1511085442
Central, MA
Learn About Top Answerers
Didn't find the answer you were looking for?
Ask a question
Popular Resources
Are there grounds to recommend coffee consumption? Recent studies perk interest.
Salt in food can hurt your heart.
Get answers to your top questions about this common — but scary — symptom
How to know when chest pain may be a sign of something else
For people with Obsessive-Compulsive Disorder (OCD), the COVID-19 pandemic can be particularly challenging.
A list of national and international resources and hotlines to help connect you to needed health and medical services.